THEORY OF ADJUSTMENT 



19 



of metal, and the alignment of the metal in the field. Since the in- 

 tensity of the earth's magnetic fields varies over the earth's surface, 

 the induced magnetism in a ship will vary with latitude, heading, and 

 heel of the ship. 



21. With the ship on an even keel, the resultant vertical induced mag- 

 netism^ if not directed through the compass itself, will create deviations 

 which plot as a semicircular deviation curve. This is true because the 

 vertical induction changes magnitude and polarity only with magnetic 

 latitude and heel and not with heading of the ship. Therefore, as long 

 as the ship is in the same magnetic latitude, its vertical induced pole 

 swinging about the compass will produce the same effect on the com- 

 pass as a permanent pole swinging about the compass. Figure 12 

 illustrates the vertical induced poles in the structures of a ship. Gen- 

 erally, this semicircular deviation will be a 5 sine curve, as shown in 

 figure 13, since most ships are symmetrical about the centerline and 

 have their compasses Tnounted on the centerline. The magnitude of 

 these deviations will change with magnetic latitude changes because 

 the directive force and the ship's vertical induction both change with 

 magnetic latitude. 



22. The masses "of horizontal soft iron which are subject to induced 

 magnetization create characteristic deviations, as indicated in figure 

 13. The D and E deviation curves are called quadrantal curves be- 

 cause they reverse polarity in each of the four quadrants. 



23. Symmetrical arrangements of horizontal soft iron may exist 

 about the compass in any of the patterns illustrated in figure 14. 



1?' 



T? 



Resultant of Vertical 

 Induced Components 

 (North Latitudes) 



FiGURB 12. — Ship's vertical induced magnetism. 



Vertical Induced 

 Components 



24. The deviations resulting from the earth's field induction of these 

 symmetrical arrangements of horizontal soft iron are illustrated by 

 the sketches in figure 15, showing the ship on a few different compass 

 headings. The other heading effects may be similarly studied. 



Such a D deviation curve is one of the several curves indicated 

 in figure 13. It will be noted that these D deviations are maxinvwm 

 on the intercardinal headings and zero on the cardinal headings. 



25. Unsymmetrical arrangements of horizontal soft iron may exist 

 about the compass in a pattern similar to one of those in figure 16. 



